I suspect the problem is cool soil. Cannas really love rich moist soil and lots of heat to grow their best. To give them a head start you could pot them up indoors in the spring to get them started, then transplant once the soil has warmed up outside, usually about two weeks or so after the last frost. Keep them just barely moist until they sprout, then increase the water as they grow. Be sure to gradually acclimate them to being outside over about a week's time before you plant them in the ground.